1. Field of the Invention
The present invention relates to an optical communication network, and more particularly to a metro wavelength division multiplexing network.
2. Description of the Related Art
An erbium doped fiber amplifier (EDFA) and a Raman amplifier are typically used for amplifying attenuated optical signals in the transmission of mass storage data in a metro optical communication network. Gain flat bands of optical amplifiers must be widen in order to employ a great number of channels during transmission. Therefore, the EDFA usually employs a gain flattening filter so as to obtain flattened gains. Also, the Raman amplifier flattens gains by coupling a plurality of pump lights of mutually differing wavelengths in order to obtain flattened gains.
As the peak of a Raman gain occurs at a long wavelength shifted from a wavelength of a pump light by about 13 THz, it is possible to select desired amplification bands according to a center wavelength of the pump light and adjust gain-flatness of an optical amplifier using a plurality of pump wavelengths. However, since the Raman amplifier generally has inferior amplification efficiency, the Raman amplifier must employ pump light sources having great power in order to output high power. Thus, the implementation of Raman amplifier is expensive.
Although there are various disadvantages, erbium doped fiber amplifiers (EDFAs) have been recently replaced with semiconductor optical amplifiers (SOAs) to be used as the semiconductor optical amplifiers. The SOA has low power and a high noise figure. Nevertheless, since the SOA can be fabricated through mass-production in a compact size, the SOA has been represented as an economical optical amplifier. Moreover, the SOA can be used in a metro wavelength division multiplexing network because the metro wavelength division multiplexing network does not require a long distance between optical repeaters, so it is not necessary for the optical amplifier to have a high-level output property.
Currently, there have been many efforts of using the SOA together with the Raman amplifier in order to compensate for the disadvantages associated with the SOA. Although the Raman amplifier has low output power, the Raman amplifier has a superior noise figure, so it is possible to overcome disadvantages of the SOA if the Raman amplifier can be used in a novel way with the SOA.
FIG. 1 is a view showing a structure of a conventional metro wavelength division multiplexing network. The network 100 includes first to third optical repeaters (ORs) 112, 114, and 116 connected to each other through an optical fiber link 160. The first to the third optical repeaters 112, 114, and 116 include Raman gain mediums (RGMs) 122, 124, and 126, a plurality of pump light sources (LSs) 131 to 139, optical couplers (CPs) 142, 144, and 146, and semiconductor optical amplifiers (SOAs) 152, 154, and 156.
The first optical repeater 112 includes the first Raman gain medium 122, the first to the third LSs 131 to 133, the first CP 142, and the first SOA 152. The second optical repeater 114 includes the second Raman gain medium 124, the fourth to the sixth LSs 134 to 136, the second CP 144, and the second SOA 154. The third optical repeater 116 includes the third Raman gain medium 126, the seventh to the ninth LSs 137 to 139, the third CP 146, and the third SOA 156. Hereinafter, the description will be made only with reference to the first optical repeater 112 because the first to the third optical repeaters have the same structures with each other.
In operation, the first to the third LSs 131 to 133 output pump lights having first to third wavelengths (λ1 to λ3), and the first CP 142 outputs the inputted pump lights (λ1 to λ3) to the first RGM 122. The first RGM 122 is pumped by the pump lights (λ1 to λ3) and also amplifies inputted optical signals so as to output the amplified optical signals. The amplified optical signals are inputted to the first SOA 152 through the first CP 142. The SOA 152 re-amplifies the inputted optical signals so as to output the re-amplified optical signals to the optical fiber link 160.
In a long distance communication, a cost of an optical communication network is less important than a reliability thereof. In contrast, in a subscriber regional network or a metro regional network, a cost of the optical communication network becomes a more important factor. However, in the conventional metro wavelength division multiplexing network, it is necessary to have a flattened gain characteristic in order to amplify optical signals having multi-channels. To this end, optical repeaters may include a plurality of pump light sources. For these reasons, manufacturing costs of the optical repeaters may increase.